This invention relates to a Magnetic Resonance Imaging (MRI) system. More particularly, this invention relates to superconducting coils used in MRI systems for correcting central magnetic field temporal shift, and for shielding external magnetic disturbances from large electromagnetic fields.
A highly uniform magnetic field is useful for nuclear resonance image (MRI) and nuclear magnetic resonance (NMR) systems as medical devices or chemical/biological devices. Most popular systems currently available worldwide use a superconducting magnet system which creates a highly uniform field in a pre-determined space (imaging volume). A superconducting magnet system usually uses multiple superconducting coils (main coil system) to achieve a desired highly uniform magnetic field in the imaging volume. More advanced superconducting MRI and NMR magnet systems also use an active shielding technique which adds a second set of multiple coils (shielding coil system) which creates a reverse direction magnetic field to reduce the fringe magnetic field and to achieve a significant reduction of the external magnetic field in the surrounding space of the magnet system. Depending on the design, the main coils system and shielding coils system can use a single circuit running the same electrical current, or two individual circuits running either the same current or two different currents. From the laws of physics, one knows that for a single superconductive closed loop, the total magnetic flux inside of the loop does not change. However, for a multi-coil system, especially for an actively shielded MRI magnet system with main coils and shielding coils connected in series, the situation is a little different.
Due to the environment disturbances, such as a train and/or other moving vehicles, rotating machinery, elevators, etc, in the surrounding area, the magnetic field of the system will have a corresponding temporal magnetic flux change. Practically, all magnet systems are subject to such temporal field instability ranging from ppm (parts per million) to ppb (parts per billion). But for actively shielded magnet systems, this change is more severe. For good image quality, the temporal field variation of a typical MRI should normally be less than 0.05 to 0.1 ppm/hour. The stability of the magnet center field is, however, highly affected by the environment disturbances, especially for those actively shielded magnets. The magnitude of the field fluctuations depends on both the size of the object and the distance away from the magnet system. For example, a typical elevator 20 feet away from the magnet can cause a field fluctuation of about 0.01 Gauss or 1.0E-6 Tesla, a subway can also cause a 0.1 Gauss field fluctuation.
Clearly, these environment disturbances included changes in both center magnetic field and its homogeneity will cause detectable deviation of the nuclear imaging quality (imaging distortion) for MRI and NMR devices.
In order to minimize such effects caused by environment changes and other disturbances, the electrical currents changing in both main coils and shielding coils should be controlled or limited to some prescribed acceptable level such that the environment disturbance is compensated and the center magnetic field remains constant and uniform. One structure and method has been described in U.S. Pat. No. 4,926,289 for such purpose by using a single filament or a few filaments of superconducting wire for the purpose of having low critical current. However, it would be desirable to provide methods and apparatus which are not constrained to filament(s) with low critical current.
In one aspect, a method of operating an imaging system having a main coil and a shield coil electromagnetically coupled to the main coil is provided. The method includes monitoring for an external environmental fluctuation of electromagnetism, and controlling current flow through the main and shield coils based upon the monitoring using a quench heater.
In another aspect, an imaging system includes a main coil, a shield coil positioned to shield an electromagnetic field generated by the main coil, and at least one environmental fluctuation circuit operationally coupled to at least one of the shield coil, the main coil the circuit including at least one detection coil, and a quench heater positioned proximate the detection coil.
In a further aspect, a method of operating an imaging system comprising a main coil, a shield coil positioned to shield an electromagnetic field generated by the main coil, and at least one environmental fluctuation circuit operationally coupled to at least one of the main coil and the shield coil, the circuit comprising at least one detection coil, and a quench heater positioned proximate the detection coil is provided. The method includes energizing the quench heater such that the detection coil is in a non-superconductive state, supplying current to the main coil and the shield coil until a predetermined current is reached while the detection coil is in the non-superconductive state, activating a persistence switch to a superconductive state, and de-energizing the quench heater when the persistence switch is in the superconductive state.